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Home , Carburetor, Ignition system, Ignition timing, Supercharger

Tuning a - Equipped Supercharged Engine

Whenever people see a carburetor-equipped supercharged engine in a hot vehicle, they can’t help noticing how impressive it looks, and they wonder how much power it produces. But they also see an engine they think is almost impossible to tune, is not very reliable, and will overheat at the drop of a hat. It doesn’t have to be that way.

by Henry P. Olsen

26 Professional www.hotrodprofessional.com This ‘56 Ford 292 Y-Block has a Paxton . That combo in a ‘57 held the NHRA L-Stock record for decades.

www.hotrodprofessional.com June 2014 27 Tuning

A supercharged engine doesn’t have to be a more capable water , or even a larger a temperamental powerhouse as long as the radiator. The -- spark advance engine is built, and tuned, to work with the curve and spark output -- must be tuned for supercharger. A properly-built supercharged the unique demands of a supercharged engine. engine will withstand the extra load on The system needs a lot more power output to bearings and other parts caused by all the fire the with, say, six psi of boost power the engine yields, and will have a than is needed on a non-supercharged engine designed to work with the with a typical 9:1 compression ratio. Of course, boost pressure and the octane of the as the boost pressure increases, the need for it will be burning. Careful attention to choices greater ignition spark output also increases. and upgrades in several areas can tame the beast almost to respectability. These include: This higher output will require upgraded • Cooling system spark plug wires, and the rest of the system must be up to the job of delivering all that • Ignition system and timing extra energy to the spark plugs. Consult the • Carburetor or supplier of the ignition system you are using • supply to determine the recommended spark plug • Air/fuel mixture gap, and any other changes needed, for a supercharged engine. You probably will want Cooling and ignition systems to do some experimenting on your own to The cooling system, for instance, will need to find the spark plug gap that works best for the be upgraded to carry enough heat away from needs of your particular engine. an engine that’s producing a lot more power -- and generating a lot more heat -- than a A standard points-type ignition system with non-supercharged engine. This may involve sufficient output to fire spark plugs with a gap

The supercharged Hemi in this ‘32 Ford runs just as great as it looks.

28 HOT ROD Professional www.hotrodprofessional.com of 0.035 in. in a non-supercharged engine may Spark retard can help need the spark plug gap to be as small as 0.018 A spark retard system designed for a in. when used with on a supercharged engine supercharged engine, such as MSD’s Boost with six psi of boost. The ideal ignition curve Timing Master, can be used to retard the for a supercharged engine advances the spark timing as the boost pressure increases. This quicker, yet provides less total spark advance, type of system allows for a more aggressive than for an engine without a supercharger. advance curve so the engine will have This quicker advance helps give the engine good initial response before the better throttle response until the supercharger supercharger boost comes in, but will retard boost pressure kicks in, but because the the spark as the boost pressure increases to supercharger increases the pressure in the avoid detonation [see Greg McConiga’s article cylinders the engine will need less total spark on the dangers of detonation in the April issue advance to avoid detonation problems. of HOT ROD Professional]. This system can allow setting the total at 36 A good starting point for a supercharged degrees, with the system retarding the spark 350 small-block Chevrolet with an 8:1 one to three degrees per pound of boost. Total compression ratio and six psi of boost is spark timing can be retarded by as much as an initial setting of 18 degrees BTDC and a 20 degrees at one degree for each pound of total of 30 degrees of advance achieved at boost pressure. Note: A supercharged engine 3,000 rpm. This ignition timing curve is just should generally not rely on vacuum advance a beginning, and must be tailored to your -- the extra spark advance it typically supplies engine package for best performance. can lead to engine damage.

The MSD Boost Timing Master system allows you to retard the timing one to three degrees per pound of boost pressure. www.hotrodprofessional.com June 2014 29 Tuning

The correct carburetor(s) Selecting the correct carburetor(s) is one of the most important choices you can make in setting up a supercharged engine because all that extra means a lot more air. If the correct air/ fuel mixture isn’t available for all operating conditions, engine damage may result.

The most notable difference in a carburetor designed to be used with a supercharger is that the power includes a vacuum tube connection that mates with the boost If you expect your supercharged engine to perform as it pressure/vacuum signal in should, the advance curve will have to be tuned the manifold below for the unique needs of “artificial respiration.” the supercharger. This makes the power valve “boost referenced” -- the it will supply the richer air/fuel mixture the engine needs when the supercharger’s boost pressure is creating more horsepower.

A standard performance carburetor’s power valve reads the vacuum at the base plate of the carburetor. If this type of carburetor is used on a supercharged

Right: The best way to check the advance curve is with a distributor test bench.

30 HOT ROD Professional www.hotrodprofessional.com engine, it will be reacting to the vacuum created by the supercharger instead of the vacuum or pressure that the supercharger is blowing into the engine. So, a power valve that is not boost-referenced will “see” a vacuum signal from the supercharger that is, in many cases, 10 in. Hg of vacuum higher than the actual engine vacuum. This will cause the power valve to stay closed because it is not reading the correct vacuum signal.

When the power valve does not open at the correct load conditions, A dial-back can also allow you to observe the the air/fuel mixture will be spark advance. too lean, this can lead to internal engine damage.

If you must use a carburetor that does not have a boost-referenced power system, select a power valve or power spring that causes this carb circuit to open when the vacuum signal drops below 10 to 14 in. Hg. A properly-tuned supercharged engine with a boost-referenced power valve will work best, in most cases, with a power valve that opens Here’s the boost-referenced vacuum port of a Holley at 4.5 in. Hg. carburetor designed for a supercharged engine. www.hotrodprofessional.com June 2014 31 Tuning

Selecting the correct air flow or cfm rating mixtures, especially in off-idle and part- of the carburetor(s) is also very important throttle driving conditions. The idle and off-idle if you expect the supercharged engine to systems on 500, 600 and 650-cfm supply maximum power and driveability. The carburetors can be modified to avoid an off- first step in selecting the right carburetor(s) is idle lean condition, so they can work well on to know how the vehicle will be driven most a low-boost single or dual carburetor set-up. of the time. If the vehicle will be driven 90% We tend to avoid the 750 and 800 cfm AFBs of the time at part throttle, select a smaller on street-driven blown engines, due to a lean carburetor(s) in the 500-650 cfm range. But part-throttle situation inherent in these carbs. if the engine will be operated at wide-open throttle 90% of the time, consider a larger cfm The fuel supply system must be capable of carburetor(s), knowing that low engine speed maintaining proper fuel pressure at all driving driveability may suffer. conditions. If the pressure drops, the air/fuel mixture will go too lean for the engine’s needs. The small amount of extra horsepower you If this happens while the engine is under will gain at higher rpm ranges by selecting a boost, the result can be damage to larger carburetor(s) may not be not be worth and rings. The , fuel line size, and the driveability you will lose at part throttle! vent system must work together to supply the carburetor(s) with enough fuel A smaller carburetor can still to support all the power the supercharged generate good power engine is capable of making. A supercharger can do a very good job of making a lot of horsepower even when the carburetor is on the small side. The Carter-designed AFB and AVS carburetors sold by Edelbrock, and the non-boost- referenced Holley-style carburetors can work well on a low-boost- pressure/mild blown engine. But when a carburetor does not have a boost- referenced power This picture shows the vacuum hose that connects to a port system, extreme care on the manifold below the supercharger that goes to the tube must be taken to behind the choke on this Holley carburetor’s boost-referenced avoid overly lean fuel power valve.

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Tuning the air/fuel mixture on a of an engine misfire. The CO2 content in supercharged engine the exhaust is the product of complete Modern technology has supplied the combustion, so the best air/fuel mixture and tools that make it possible, at an affordable the ideal ignition spark timing will create the cost, to properly tune the air/fuel mixture highest CO2 reading. NOx content in the of a supercharged engine. One of the best exhaust is created by excessive combustion methods for determining what the air/fuel chamber heat, and this gas can be seen mixture (jetting) is -- on any engine -- is the as a precursor to detonation, and used as use of a portable five-gas infrared exhaust a warning. The O2 or reading can gas analyzer. This measures the critical gases indicate a lean air/fuel mixture, an exhaust in the exhaust, then supplies readings that leak, that the engine has a “hot” -- or it can show combustion efficiency, misfire, air/ could be the result of the supercharger “blow- fuel ratio, and excessive through” effect. heat (detonation). Note: If O2 content readings in the exhaust The CO reading from an are above two to three percent, the dilution of analyzer is a very accurate indicator of the the exhaust gases being measured can cause air/fuel mixture that the engine is getting. The the accuracy of all of the gas readings -- or HC reading indicates the amount of unburned wide-band -based digital air/ fuel in the exhaust -- too much is an indicator fuel ratio meter readings -- to be suspect.

The air/fuel mixture can also be checked using a wide-band oxygen sensor- based digital air/fuel ratio meter. This method reads the oxygen/unburned combustibles content in the engine exhaust, then supplies an air/fuel mixture reading. The readings are normally very accurate, but this type of meter is subject to false readings created by the supercharger blow- through effect, exhaust leaks, engine misfire, or the valve overlap of a high- Barry Grant Road Demon Jr. blower carbs use a connection performance at on the base plate below the fuel bowls to supply the power low engine rpm and low- with the correct vacuum signal for the needs of a load conditions. supercharged engine.

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False lean readings a vacuum reading of 12 in. Hg of vacuum Any extra oxygen seen by the wide-band above the supercharger -- at the carburetor. oxygen sensor as it looks at the exhaust will But a vacuum reading below the supercharger cause the meter to display a “false lean” where the boost-referenced power valve gets reading. A digital air/fuel ratio meter is a its reading -- in the intake manifold -- may very good, highly-portable, and low-cost be two to three in. Hg of vacuum. This would method of determining what the air/fuel mean a boost-referenced power valve would mixture is while the is driven on the road be supplying the correct power mixture, but at any condition from a light-load-cruise to a non-boost-referenced power valve would a high-power, wide-open-throttle run down supply a much leaner mixture, which could the race track. lead to detonation and/or engine overheating.

A good power mixture for most low-boost The target cruise mixture normally used on engines is indicated by a CO reading of a mild-cam engine with low boost pressure 6.6% -- 12:1 air/fuel ratio -- while a higher- would be 1.0% to 2.0% CO -- 14.1-13.8:1 air/ boost supercharged engine might need a CO fuel ratio -- but on a hot-cam, high-boost reading of 8.0% -- 11.5 to 1 air/fuel ratio -- or engine the target mixture we use is 3.0% CO, richer. This richer mixture can help control representing a 13.4:1 air/fuel ratio. detonation created by the high boost pressure, but if it is too rich the engine will lose power. As mentioned earlier, a good starting place for power valve opening on a boost- referenced power system is 4.5 in. of vacuum -- this allows the carburetor to supply the richer power mixture needed by the engine when the driver opens the throttle.

Boost-referenced carburetors avoid lean air/fuel mixture A typical big-block Chevrolet engine with a 6-71 supercharger in a The vacuum tube on this prototype spacer would be used to street rod accelerating down the road may have supply the power system of a Carter-designed AFB or AVS carburetor with a boost-referenced vacuum signal.

36 HOT ROD Professional www.hotrodprofessional.com Power, performance, and appearance -- some people prefer the old-school look of this ‘32 Ford, but if you prefer new-school you could always supercharge your late-model Corvette. Fuel pressure is a concern with blow-through The first concern for an engine using a blow-through supercharger -- where the carburetor is under boost -- is fuel pressure. A special regulator that is boost- referenced, along with a high pressure/high volume fuel pump, is mandatory. The regulator senses the amount of boost and keeps the fuel pressure at a set point above the boost pressure. The setting that we see most often is 5.5 to 6.5 psi over the boost pressure. For example, with six psi of boost, the fuel pressure would need This flathead Ford has Ardun heads and a supercharger that to be set at 11.5 to 12.5 breathes through two Stromberg 97 carburetors. www.hotrodprofessional.com June 2014 37 Tuning psi. The two methods of pressurizing or blowing as it blows into the carburetor and its air through the carburetor are: bleeds -- if some method of diffusing the • the carburetor is in a pressurized box air-charge is not used, the air flow blowing from just one side of the carburetor across • a “hat” is used on top of the carburetor in the air bleeds may cause air/fuel mixture place of the air cleaner. curve problems, possibly leading to engine If a hat is used, the throttle shafts will need damage from an overly lean or rich mixture to have seals installed to keep the fuel from in one or more cylinders. blowing out through the clearance between the shaft and its . Every carburetor we So that’s the story -- you can drive an are aware of was designed to work with the awesome, supercharged machine that air flow coming in evenly from all directions scorches tires at the touch of the throttle, as it travels through the air cleaner. But when but that is relatively well behaved when you the air is blown through with a supercharger, want it to be. Match the supercharged engine it may not supply all of the emulsion bleeds in with the correct carburetor(s) properly tuned the carburetor with the same air pressure or and well-supplied with fuel, and an ignition flow patterns. system with the right parts and spark curve, and you have built a supercharged driving A marine-type flame arrester inside the air experience that is both enjoyable and box or hat can help diffuse the air charge trouble-free. n

Yes, you can have it all: power, performance, and that supercharged look. All it takes is the time to pick the correct parts and properly tune it for the special needs of a supercharged engine.

38 HOT ROD Professional www.hotrodprofessional.com